Method and apparatus for reducing warpage during application and curing of encapsulant materials on a printed circuit board

ABSTRACT

A method and apparatus for preventing board warpage during the application and curing or drying of liquid epoxies, or the like, on printed circuit boards using a clamping fixture assembly, which includes at least one clamping fixture support and at least one clamping fixture overlay. If desired, a plurality of printed circuit boards may be processed using an appropriate clamping fixture assembly. Furthermore, the clamping fixture may be constructed so a slight bow or curvature thereof can counter either a convex or concave bow or curvature of the printed circuit board. 
     In the method, at least one printed circuit board is mounted to a clamping fixture support whereby a clamping fixture overlay is placed on top of the first printed circuit board.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/578,255,filed May 24, 2000, now U.S. Pat. No. 6,592,670, issued Jul. 15, 2003,which is a divisional of application Ser. No. 09/170,628, filed Oct. 7,1998, now U.S. Pat. No. 6,224,936 B1, issued May 1, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to preparing printed circuitboards for the mounting of semiconductor devices thereon. Moreparticularly, the present invention is directed to the preparation andfabrication of printed circuit boards to reduce warpage caused by theapplication of epoxy encapsulant material placed upon the surface of theprinted circuit board.

2. State of the Art

The fabrication of integrated circuits on areas of a wafer to form adiscrete semiconductor die thereon is a long and complex process. One ofthe last steps in the process is that of encapsulating the semiconductordie as a semiconductor device and attaching the die to a printed circuitboard (PCB) or other type die carrier.

Conformal coatings and encapsulants are typically applied as one of thelast major processes of the fabrication of either the printed circuitboard or other type of die carriers. In either case, the combination ofthe semiconductor device attached to a printed circuit board or othertype die carrier has increased the value of the assembly over the valueof the separate components. Therefore, the mounting of the semiconductordevice and the encapsulation thereof on the printed circuit board orother type die carrier must have a high reliability and high yield,respectively.

Encapsulation of the semiconductor device protects the semiconductordevice during any subsequent processing and prevents mechanical damagewhile providing protection from the operating environment for thesemiconductor device.

Conformal coatings are used to encapsulate and protect various types ofelectronic packages, primarily from their operating environments.Specialized coatings have been developed to provide an enhancedprotection from direct attack of hostile gases and liquids on criticalsurfaces of the packages. Polymeric films act only as semi-hermeticbarriers because of reduced solubility or permeability of a hostilereactant in the polymeric material, or both.

Polyamides, polyamide-imids, and silicones have been developed forapplications that can tolerate high cure temperatures and that needprotection at elevated temperatures. These types of materials are mostfrequently used either directly on the semiconductor die at the dielevel, as passivating layers thereon or at the die carrier level.Polyurethanes, fluoropolymers, silicones, accrolades, and epoxies aremost commonly used for components and printed circuit boards. Thesematerials are typically applied from solution by emersion or spraycoating or they may be applied via stencil coating or direct spreading.After curing, most coatings used on the semiconductor devices and aprinted circuit board or other type carrier are difficult to removebecause they become cross-linked during the curing process.

Materials typically used to encapsulate semiconductor dice mounted onvarious types of lead frames and to seal metal cans housingsemiconductor dice and their carrier, as well as many other components,including potting and molding compounds as well as glob topencapsulants, must provide protection from handling damage for thesemiconductor dice and their carrier in the post processing environmentand any subsequent operating environment. Semiconductor dice are mostfrequently electrically connected to the lead frame by bonding wiresbetween the bond pads on the semiconductor die and the leads of the leadframe (wire bonding). Flip-chips use small solder balls as interconnectsto a substrate and tape automated bonding using thermal compressionbonding to form interconnections between the circuits located on thetape and the bond pads of the semiconductor die. Interconnectionsbetween substrates and semiconductor dice, as well as other components,are fragile and subject to stress failures. The encapsulant must notgenerate catastrophic stresses due to the chemical curing process of theencapsulant material or stresses due to differing rates of thermalexpansion of the semiconductor die, substrate, and encapsulant duringthe thermal cycling thereof.

Initially, rigid epoxies were primarily used for encapsulation. Epoxieshave the advantages of relatively little shrinkage, high resistance toprocessed chemicals, and good mechanical properties. Since semiconductordevice package sizes are growing, highly filled epoxies with reducedthermal expansion have been developed to reduce stresses in thesepackages.

Unfortunately, even the best of epoxies still has some level ofshrinkage that results in warpage of the underlying substrate, such as aprinted circuit board (PCB). The warpage of a printed circuit board canstress the board enough to either cause it to fail or to cause any ofthe attached semiconductor devices to fail. Failure of semiconductordevices typically occurs because the solder links between thesemiconductor device and the circuits on the printed circuit boardfailed due to the stress caused by the warpage of the board. Conformalcoatings may also incur stress on a surface mounted chip (SMC) duringthermal cycling of the chip and printed circuit board causing the solderjoints to crack or the components to fracture. Differences between thecoefficients of thermal expansion of the encapsulant, the coating, theprinted circuit board, and a semiconductor device mounted thereon causegreater stress during thermal cycling. A coating that has a coefficientof thermal expansion (CTE) nearly matching that of the substrate and thesemiconductor devices mounted thereon will produce less stresstherebetween and attendant cracking when subjected to thermal cycling.Larger surface-mounted chips are more vulnerable to damage from stressesduring curing of the encapsulant material and thermal cycling of thechip and substrate due to the differences in the coefficient of thermalexpansion of the chip and substrate causing stresses therebetween.

The thicker the coating or encapsulant thickness of a semiconductordevice, the greater the likelihood of stress on the semiconductor deviceand its connections or interconnects to the substrate from shrinkage ofthe coating or encapsulant. Some surface-mounted chips may not be ableto withstand mechanical stresses induced during curing of thickcoatings, which may also result in the warpage of the printed circuitboard upon which the chip is mounted. If the solder interconnectionsbetween a semiconductor device and the circuits of a printed circuitboard are closely spaced, conventional coating materials and encapsulantmaterials may move the semiconductor device, thereby cracking the solderjoints as such material cures. In addition, thicker material coatings orthicker encapsulant material may act as barriers to heat transfer fromdensely packed surface mount chips during the operation thereof.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus forpreventing board warpage during the application and curing or drying ofliquid epoxies, or the like, on printed circuit boards. A clampingfixture assembly, which includes at least one clamping fixture supportand at least one clamping fixture overlay, is used to restrain theprinted circuit board on a flat surface during the curing of the epoxy.If desired, a plurality of printed circuit boards may be processed usingan appropriate clamping fixture assembly. Furthermore, the clampingfixture may be constructed so a slight bow or curvature thereof cancounter either a convex or concave bow or curvature of the printedcircuit board.

In the method, at least one printed circuit board is mounted to aclamping fixture support where a clamping fixture overlay is placed ontop of the first printed circuit board. Next, an aperture in theclamping fixture overlay allows for the application of an encapsulationmaterial, such as an encapsulant epoxy, to be spread within an areabordered by an epoxy dam. Next, the epoxy is cured or dried on theprinted circuit board. Such curing or drying can be performed within anoven for a predetermined period of time at a predetermined temperaturesufficient to optimize curing or drying of the epoxy without excessiveboard warpage, such warpage being limited by the printed circuit boardbeing retained in the clamping fixture assembly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a clamping fixtureapparatus according to the present invention;

FIG. 2 is an exploded perspective view of a clamping fixture apparatusthat allows more than one printed circuit board to be encapsulated;

FIG. 3 depicts the problem of printed circuit board warpage;

FIG. 4 depicts the end result of a printed circuit board having epoxythereon cured with minimal board warpage;

FIG. 5 depicts an alternative clamping fixture that uses a channel forholding a printed circuit board in place;

FIG. 6 depicts a single-inline-module (SIMM) having a plurality ofsemiconductor dice; and

FIG. 7 depicts a perspective view of a multiple dam clamping fixtureapparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to drawing FIGS. 1 and 3, a perspective diagram depicts afirst embodiment of a clamping fixture assembly apparatus 10 of thepresent invention that is used to prevent a printed circuit board fromwarping during application and curing of epoxy resins used during theencapsulation or sealing of the semiconductor devices mounted on theprinted circuit board. The warpage of a printed circuit board 36 havinga semiconductor device 46 located thereon is illustrated in drawing FIG.3. The clamping fixture assembly apparatus 10 includes a support base 12and one or more clamping elements 16. A printed circuit board 14 havingat least one semiconductor device mounted thereon is mounted on thesupport base 12. A plurality of clamping elements 16 is mounted upon thesurface of support base 12 along the edge region of printed circuitboard 14. A plurality of threaded fasteners, retaining elements 18, isused to retain each clamping element 16 on the support base 12 so as toapply a downward force along the opposing edges of printed circuit board14. The support base 12 may be substantially flat or may have a slightradius of curvature that may be either concave or convex. Concaveapplications are useful in removing a convex bowing of a printed circuitboard 14 while a convex surface would be helpful in eliminating aconcave bowing of a printed circuit board 14. Once a printed circuitboard 14 is placed and retained securely within clamping fixtureassembly apparatus 10, an epoxy encapsulant material 20 may be spreadacross a portion of the surface or the entire surface of the printedcircuit board 14 and then allowed to cure. The curing stage may includedrying and curing in an ambient temperature, or the entire clampingfixture assembly apparatus 10 having printed circuit board 14 mountedthereon may be placed within a curing oven to cure at a predeterminedtemperature for a predetermined length of time. Higher temperaturestypically lead to greater warpage so an optimal range of time andtemperature that minimizes shrinkage of the epoxy during the curingthereof may be determined for the desired epoxy material. Additionally,the epoxy material used may be selected so that it has a coefficient ofthermal expansion (CTE) that closely matches that of printed circuitboard 14.

The printed circuit board 14 may be selected from any number ofelectronic substrate materials such as, for example, fiber reinforcedboard number 4(FR4), ceramic substrates, metal clad fiber boards, or anyother type of rigid substrate material that tends to warp either duringthe manufacture of the PCB or during the curing of the epoxyencapsulants.

After sufficient time has elapsed to cure epoxy 20, the clampingelements 16 are then removed by removing the retaining elements 18 fromclamping elements 16 and support base 12. Once the clamping elements 16have been removed, the printed circuit board 14 may be removed.

Referring to drawing FIG. 2, a second embodiment of the invention isdepicted in a perspective exploded view of a clamping fixture assemblyapparatus 30 that is used for holding an array, more than one, ofprinted circuit boards and also serves as an epoxy dam for theapplication of the epoxy across the surface of the printed circuitboards. Clamping fixture assembly apparatus 30 includes a support base32 and a clamping element 34 is secured to support base 32 by retainingelements 42. A plurality of printed circuit boards 36 is placed upon thesurface of support base 32 in such a position so as to be exposed by damapertures 38 that are formed within clamping element 34 when attached tosupport base 32. A plurality of receiving apertures 40 is located in theouter perimeter corners of support base 32 and is aligned with retainingelements 42 that insert into recieving apertures 40 in support base 32when the clamping element 34 mates with support base 32. Once clampingelement 34 is securely attached to support base 32, the printed circuitboards 36 are retained and exposed through dam apertures 38 in clampingelement 34. Next, an epoxy material 44, or any other type of sealant, isapplied through dam apertures 38, the perimeter of which serves as anepoxy dam during the application of the epoxy to the surface of printedcircuit boards 36. Next, the clamping fixture assembly apparatus 30having printed circuit boards contained therein is cured at ambienttemperature, or may be cured in an oven to decrease the curing time ofthe epoxy. While being retained within a secured clamping fixtureassembly apparatus 30, the epoxy 44 on the printed circuit boards 36cures with minimal warpage of the printed circuit boards 36.

Referring to drawing FIGS. 3 and 4, illustrated is a cross-sectionalschematic diagram of a printed circuit board 36 having a semiconductordevice 46 located thereon. As can be seen in FIG. 4, printed circuitboard 36 is substantially planar, having no warpage curvature thereof tocause problems. In contrast, as illustrated in drawing FIG. 3, a printedcircuit board 36 having a semiconductor device 46 located thereonexhibits substantial warpage. Returning to drawing FIG. 4, for example,if printed circuit board 36 were a single-inline-memory-module (SIMM),any warpage found in printed circuit board 36 would prevent the printedcircuit board 36 from being placed within a single-inline-memory-module(SIMM) by an automated machine for use on a motherboard having a SIMMsocket or slot thereon. In other words, the warpage exhibited in printedcircuit board 36 would require the manual insertion thereof in asingle-inline-memory-module to align within the substantially parallelchannels of circuits on a motherboard. Additionally, once printedcircuit board 36 is straightened during the insertion process, theflexing of the printed circuit board 36 into a planar configurationwould induce stress therein and possibly fracture any electrical ormechanical bonds between semiconductor device 46 mounted on the printedcircuit board 36 and the printed circuit board 36. In greatlyexaggerated warpage, the likelihood of any fractures of the bondsbetween the semiconductor device 46 and printed circuit board 36 orstressing of the printed circuit board 36 would be even greater. Otherreasons for controlling or reducing warpage in the printed circuit board36 are to facilitate subsequent process steps involving the printedcircuit board 36 by having a greater degree of planarity of the printedcircuit board 36 for dicing, marking, labeling, or the addition of othercomponents to the board, particularly where solder reflow processing isrequired, etc.

Again referring to drawing FIG. 4, a printed circuit board 36 isillustrated that has been cured in a substantially flat, planarcondition without any substantial warpage thereof. It is significantthat printed circuit board 36 may have the warpage prior to theencapsulation step and that the curing of the epoxy may be used toremove the warpage of printed circuit board 36 and/or that the clampingfixture assembly apparatus 10, 30 may have either a convex or concavecurvature thereof to offset and counteract the warpage of the printedcircuit board 14, 36 during the curing of the epoxy.

In order to determine the advantages of clamping a printed circuit board14, 36 during encapsulation of semiconductor devices mounted thereon, aseries of tests was performed using various encapsulation materialshaving various dispensing weights and various curing temperatures tocompare printed circuit boards having no use of a fixture to retain theboard and a printed circuit board retained in a clamping fixtureassembly apparatus as described herein. A 16 megabyte semiconductor diemounted in a chip-on-board configuration (COB) in asingle-inline-memory-module (SIMM) board was utilized as the baseline orstandard printed circuit board. An Asymtek 402b gantry glob top systemwas utilized to dispense a centralized rectangular pattern of Hysol 4451material in a dam configuration on the single-inline-memory-module(SIMM) board with the dam configuration having a surface dimension of2.34 inches by 0.60 inches. The dam material was dispensed at a weightof 0.15 grams. The dam was allowed to cure for one hour at 150° C. in anassembly clean room burn-in oven. After the rectangular dam had beencreated on the single-inline-memory-module (SIMM) board, a glob top filmmaterial was dispensed into the dam region under an array of variousprocess conditions as noted.

The glob top materials were selected from Hysol 4450, which has astandard coefficient of thermal expansion (CTE) of 19, Hysol CNB558-13,which has a (CTE) of 12, and TraBond FS503, which has a (CTE) of 35.Each of these glob top materials was applied into the rectangular damregion at various dispense weights ranging from 0.8 grams to 1.6 grams.The thickness of the material was held constant at 0.040 inches.Different cure temperatures and times were also tested. A first curetemperature of 165° C. was used with a time of 45 minutes and a secondcure temperature of 120° C. was used for 150 minutes. Both a free stateof the single-inline-memory-module (SIMM) board and a restrained stateof the single-inline-memory-module (SIMM) board in a clamping fixtureassembly apparatus were observed during the curing of the glob topmaterial at the predetermined curing temperatures and curing times.

After the glob top encapsulation had been completed, an opticalcomparitor was used to measure the deflection of thesingle-inline-memory-module (SIMM) board to determine the board warpage.A test fixture was made to screw down one end of thesingle-inline-memory-module (SIMM) board to an aluminum block and thenthe other end of the board was allowed to bow upward freely. Thequantitative value of board deflection was then measured from the top ofthe aluminum block to the bottom of the single-inline-memory-module(SIMM) board.

At a dispense weight of 1.4 grams, having a thickness of approximately0.040 inches, the TraBond FS503 had a free deflection of 140 mils. forthe single-inline-memory-module (SIMM) board and a restrained deflectionfor the single-inline-memory-module (SIMM) board of approximately 95mils. when cured for 45 minutes at 165° C. When cured at 120° C. for 150minutes, the TraBond FS503 reduced the free deflection for thesingle-inline-memory-module (SIMM) board of 100 mils. and a restraineddeflection for the single-inline-memory-module (SIMM) board of 70 mils.The Hysol 4450 epoxy when cured at 165° C. for 45 minutes resulted in afree deflection for the single-inline-memory-module (SIMM) board ofnearly 130 mils. and a restrained deflection for thesingle-inline-memory-module (SIMM) board of approximately 65 mils. Whencured at 120° C. for 150 minutes, the Hysol 4450 epoxy resulted in afree deflection for the single-inline-memory-module (SIMM) board of 95mils. and a restrained deflection for the single-inline-memory-module(SIMM) board of 48 mils. Further, the Hysol 558-13 epoxy when cured at165° C. for 45 minutes resulted in a free deflection for thesingle-inline-memory-module (SIMM) board of approximately 80 mils. and arestrained deflection for the single-inline-memory-module (SIMM) boardof 45 mils. The Hysol 558-13 material when cured at 120° C. for 150minutes resulted in a free deflection for thesingle-inline-memory-module (SIMM) board of 25 mils. and a restraineddeflection for the single-inline-memory-module (SIMM) board of about 18mils.

All of the variables heretofore noted do contribute in some degree tothe warpage of the printed circuit board during curing of an encapsulantmaterial, glob top material, etc. To improve the results and therebyminimize warpage of the printed circuit board during curing of anencapsulant material, glob top material, etc., it is important to matchthe coefficient of thermal expansion (CTE) of the material to that ofthe printed circuit board. For example, a glob fill coefficient ofthermal expansion (CTE) of 12 obtains better results when used with hightemperature FR4 boards having a thickness of 0.50 inches. Additionally,the amount of material dispensed across the surface of the printedcircuit board is also directly proportional to the amount of warpage.Further, a lower cure temperature of the material significantly reducesboard warpage and restraining the printed circuit board (PCB) during thecure process of the material dramatically reduces the warpage of theboard.

Referring to drawing FIG. 5, depicted is a cross-sectional schematicdiagram of an alternative third embodiment of the clamping fixtureassembly apparatus of the present invention used to eliminate or reducewarpage in a printed circuit board (PCB) that has encapsulant material,such as an epoxy, applied to the surface thereof. Clamping fixtureassembly apparatus 50 is formed in a u-shaped configuration includingrails for restraining the edges of the printed circuit board whilecuring the encapsulant material located thereon and during anyprocessing of the board. Clamping fixture assembly apparatus 50 includesopening 52, which accommodates a suitable printed circuit board 36 whichmay be contained therein having retaining shoulders 54 clamping theedges of the printed circuit board 36. Opening 52 has a thickness 56substantially matching that of the thickness of printed circuit board36. Again, clamping fixture assembly apparatus 50 may have a slightradius of curvature, either convex or concave, to induce or counter acurvature in the printed circuit board 36. Once printed circuit board 36is removed from the clamping fixture assembly apparatus 50, the printedcircuit board 36 will flex, bow, or curve in the opposite direction ofthe induced curvature by the clamping fixture assembly apparatus 50.Thus, the counter bow or curvature of the clamping fixture assemblyapparatus 50 tends to offset, counteract, or nullify any warpage causedby the shrinkage during the curing of encapsulation material, such asepoxy, resulting in a substantially flat or planar printed circuit board36 having no substantial warpage thereof.

Referring to drawing FIG. 6, a single-inline-memory-module (SIMM) 60 isillustrated. The SIMM 60 includes a plurality of semiconductor devices62 mounted on a substrate, a printed circuit board 14, having apertures66 therein and having electrical circuits 64 extending along one edgethereof. The semiconductor devices 62 on the printed circuit board 14may be encapsulated with a suitable material being applied and curedwhile the printed circuit board 14 is retained in a clamping fixtureassembly apparatus such as described herein to help control the warpageof the printed circuit board 14.

Referring to drawing FIG. 7, a clamping fixture assembly apparatus 70 isillustrated including a support base 72 and clamping element 74. Thesupport base 72 includes a plurality of apertures 76 therein forreceiving portions of clamping element (not shown) therein to retain theclamping element 74 thereon. The clamping element 74 includes aplurality of rectangular dam apertures 80 therein for the application ofa suitable encapsulant material therein with the edges of the apertures80 serving as dams to retain the material therein to cover asemiconductor device located on substrate or a printed circuit board(not shown) and a plurality of apertures 78 for receiving portions of aretaining element therein to retain the clamping element 74 on thesupport base 72 for the application and curing of an encapsulant to theboard for any subsequent processing of the board, such as the dicingthereof.

The use of a clamping fixture assembly apparatus of the presentinvention as described herein eliminates substantial subsequent warpagecaused to a printed circuit board during curing of an encapsulantmaterial, such as the curing of epoxy, and allows for easier handling ofthe printed circuit board during any subsequent processing thereof. Suchsubsequent processing of the printed circuit board may include adiscrete dicing of the board into portions having a discretesemiconductor device located thereon. The machine utilized to performsuch dicing of the printed circuit board typically requires asubstantially flat printed circuit board for dicing operations and anywarpage of the board may cause errors during the dicing process.Additionally, the use of a mold dam to limit the encapsulant material toa desired region on a printed circuit board substantially reduces oreliminates the intermediate step of applying an encapsulant material,such as an epoxy encapsulant, requiring the use of masks or stencils forthe application of the material to areas of the board. The one-stepapplication of an encapsulant material, such as an epoxy, while theprinted circuit board is clamped in a clamping fixture assemblyapparatus of the present invention allows for controlled application ofthe material and the curing thereof without further undue handling ofthe printed circuit board during the application and curing stages ofthe material. This results in a more consistent and uniform applicationand curing of materials to the printed circuit board. Additionally, whenthe substrate material, such as a printed circuit board, is held havinga substantially uniform planar or flat surface thereon, an entire chipwafer may be applied to the surface of the substrate and cured withminimal damage because of the effect of the clamping fixture assemblyapparatus of the present invention. This allows for the dicing of thesubstrate having semiconductor devices mounted thereon to beconsistently, accurately and reliably performed with minimal error andloss of the diced substrate and semiconductor devices thereon.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions, and substitutions arepossible without departing from the scope and spirit of the invention asdisclosed in the accompanying claims.

What is claimed is:
 1. A fixture for holding a substrate during thecuring of a material applied to a portion thereof, comprising: a basehaving a top surface upon which the substrate is mounted, the topsurface having a radius of curvature including one of a convex radius ofcurvature and a concave radius of curvature for mounting the substratethereon; a plurality of clamping elements, each clamping element havinga portion for engaging at least a portion of a top surface of thesubstrate, each clamping element mounted upon the top surface of thebase along at least a substantial portion of an edge region of thesubstrate for conforming the substrate to the top surface of the base;and a plurality of retaining elements for securing the plurality ofclamping elements to the base and for causing the plurality of clampingelements to having portions thereof to engage the substrate during thecuring of the material.
 2. The fixture of claim 1, wherein the baseincludes a plurality of apertures therein, each aperture for receiving aportion of a retaining element of the plurality of retaining elements.3. The fixture of claim 1, wherein each clamping element of theplurality includes at least one aperture therein for receiving a portionof a retaining element of the plurality therein.
 4. The fixture of claim1, wherein each clamping element of the plurality comprises an elongatedmember.
 5. A fixture for holding a substrate having a liquid materialthereon, comprising: a base having a top surface for mounting thesubstrate thereon, the top surface having a radius of curvatureincluding one of a convex radius of curvature and a concave radius ofcurvature; a clamping element engaging a top surface of the substratefor substantially conforming the substrate to the top surface of thebase as the clamping element engages a portion of the top surface of thesubstrate the clamping element for mounting upon a portion of the topsurface of the base along at least a substantial portion of an edgeregion of the substrate; and at least one retaining element securing theclamping element to the base and causing a portion of the clampingelement to engage a portion of the substrate after the liquid materialhas been applied thereto.
 6. The fixture of claim 5, wherein theclamping element includes at least one aperture therein for exposing aportion of the substrate.
 7. The fixture of claim 5, wherein theclamping element includes a plurality of apertures therein for exposingportions of the substrate.
 8. The fixture of claim 5, wherein thesubstrate includes at least one aperture therein receiving a portion ofthe at least one retaining element therein and wherein the clampingelement includes at least one aperture therein receiving a portion ofthe at least one retaining element extending therein.
 9. The fixture ofclaim 5, wherein the at least one retaining element includes a pluralityof retaining elements.